The present invention relates to an active matrix type display device and a display method thereof. The active matrix type display device means a display device in which pixels are arranged at respective intersecting points of a matrix, every pixel is provided with a switching element, and image information is controlled by on/off switching of the switching elements. Examples of display media for the active matrix type display device are a liquid crystal, plasma and other bodies or states whose optical characteristic (reflectance, refractive index, transmittance, light emission intensity, or the like) can be changed electrically. The invention particularly relates to an active matrix type display device which uses, as the switching element, a three-terminal element, i.e., a field-effect transistor having the gate, source and drain.
In describing the invention, the term xe2x80x9crowxe2x80x9d of a matrix means a structure in which a signal line (gate line) that is disposed parallel with a row concerned is connected to the gate electrodes of transistors belonging to the row. The term xe2x80x9ccolumnxe2x80x9d means a structure in which a signal line (source line) disposed parallel with a column concerned is connected to the sources (or drains) of transistors belonging to the column. A circuit for driving the gate lines and a circuit for driving the source lines are called a gate driver and a source driver, respectively.
Flat panel displays (FPDs) have been developed as new display devices to replace a CRT display. The active matrix type display device is typical of those flat panel displays. In the active matrix type display device, a screen is divided into pixels and the individual pixels are provided with respective switching elements, which control display information that is retained by the pixels. A typical example of the active matrix type display device is a thin-film transistor (TFT) active matrix display using a TN (twisted nematic) liquid crystal.
In this display device, the display medium is the TN liquid crystal and the image information is voltages of the pixels. That is, the transmittance of the TN liquid crystal (display medium) is controlled by a voltage retained by each pixel. Conventionally, in this type of active matrix type display device, an image is rewritten by updating display contents of all the pixels by top-to-bottom sequential scanning of rows. The image rewriting is performed at a frequency of every frame, i.e., 30 to 60 times per second (30-60 Hz).
However, for certain types of display contents, the image rewriting of such a frequency is not always necessary. For example, a still image need not be rewritten until voltages retained by the pixels decrease to such low values as cannot provide sufficient display quality. Even in the case of moving images, not all the pixels display different image information every time.
The image rewriting requires output of signals, which is a factor of increasing power consumption and, therefore, an obstacle to portable applications.
The present invention has been made in view of the above circumstances, and has an object of-reducing power consumption by making the frequency of image rewriting as low as possible in an active matrix type electro-optical device.
To attain the above object, the invention is characterized by the following steps.
First, a signal to be applied to the pixels of a certain row is compared with a corresponding signal of the immediately previous frame. A signal (refresh pulse) indicating the necessity of rewriting is output only when the two signals are different for at least one pixel of the row concerned. The difference between the two signals (which are, for example, an input signal and an output signal of a delay circuit) is detected by comparing the two signals in the delay circuit.
The rewriting is then effected by applying a gate pulse to a gate line of the row concerned by using the refresh pulse, to thereby make the gate electrodes of active matrix transistors of the row concerned in an on state.
If a signal to be applied to the pixels of the row concerned is the same as a corresponding signal of the immediately previous frame for all the pixels, no refresh pulse is issued as a general rule. However, if a state in which image information is kept completely the same continues over a very large number of frames, no execution of rewriting for such a long time causes various problems. For example, where a TN liquid crystal is used as the display medium, application of a voltage of the same polarity for a long time causes an electrolysis, resulting in its deterioration. Therefore, polarity inversion needs to be performed regularly. Where only a single transistor is used as the active matrix switching element, image information (for instance, a voltage) stored in a pixel is varied by a source-drain leak current etc.
Considering the above, in the invention, rewriting to pixels is forcibly effected one per several frames even if no change occurs in image information. Where a liquid crystal material is used as the display medium, it is favorable that the polarity of voltages applied to the liquid crystal be inverted (applying AC voltages) in the process of forcibly effecting the rewriting to pixels.
In the above manner, power consumption can be reduced by decreasing the frequency of image rewriting as a whole by effecting the rewriting to only pixels or rows which need the rewriting. To avoid deterioration of display characteristics, it is effective that the regular rewriting be effected in the following manner.
Assume a matrix that is composed of 20 rows, i.e., a 1st row, 2nd row, 3rd row . . . , 19th row and 20th row. It is also assumed that completely the same image continues to be displayed by this matrix, and that forcible rewriting is performed once per 5 frames.
The simplest scheme is to perform rewriting to all the rows in the first frame and perform no rewriting in the second to fifth frames. However, in this scheme, the brightness varies during the second to fifth frames by such phenomena as reduction of pixel voltages. The same brightness as in the first frame is restored by rewriting in the sixth frame.
If the one-frame period is 30 msec, the interval between two rewriting operations is 150 msec. Therefore, a brightness variation due to the rewriting in the sixth frame is sufficiently recognizable, as a flicker, to the naked eye.
This problem can be solved by distributing rewriting operations to the first to fifth frames rather than effecting the rewriting only in the first frame. More specifically, four rows are subjected to rewriting in one frame. For example, in the first frame, rewriting is forcibly performed on only the 1st row, 6th row, 11th row and 16th row. In the second frame, rewriting is performed on the 2nd row, 7th row, 12th row and 17th row. In the third frame, rewriting is performed on the 3rd row, 8th row, 13th row and 18th row. In the fourth frame, rewriting is performed on the 4th row, 9th row, 14th row and 19th row. In the fifth frame, rewriting is performed on the 5th row, 10th row, 15th row and 20th row. The similar operations are performed in the sixth frame onward. Rewriting operations may be allocated in a different manner according to the same principle.
Stated more generally, where the entire matrix is divided into N groups each consisting of m rows, N rows are subjected to forcible rewriting in one frame and rewriting to the entire rows is completed in m frames.
In this case, for example, the above-mentioned 1st row may be referred to as a first group, first row; the above-mentioned 7th row as a second group, second row; the above-mentioned 14th row as a third group, fourth row; and the above-mentioned 20th row as a fourth group, fifth row. On the other hand, the groups and rows may be given numbers in different manners.
It is possible to make a flicker not recognizable by distributing forcible rewriting operations in the above manner. As a typical example, there is a rule that in the (kxe2x88x921)th frame counted from a frame next to the frame (called the first frame) in which the first row of each group is subjected to forcible rewriting, i.e., in the kth frame (k=1, 2, 3, . . . , m), the kth row should be subjected to forcible rewriting. The above-described example satisfies this rule.
However, it is not required at all to satisfy such regularity. It is sufficient to satisfy a rule that in m consecutive frames, forcible rewriting should be performed in one frame on one row of a gate line group consisting of m arbitrary rows, and all the rows of that group should be subjected to rewriting.
If the invention is viewed in a different way, it is understood that it is sufficient to satisfy a rule that in the mth frame counted from a frame next to the frame (called the first frame) in which a certain row is subjected to forcible rewriting, i.e., in the (m+1)th frame, the same row should again be subjected to forcible rewriting.
Further, where a liquid crystal material is used as the display medium, it is favorable that the polarity of voltages applied to the pixels of a row concerned in the (m+1)th frame be opposite to the polarity of voltages applied to the same pixels in the first frame and the (2 m+1)th frame. This is so because utilizing such forcible rewriting the liquid crystal material can be supplied with indispensable AC voltages.